A single-phase permanent split-capacitor (PSC) induction motor is often used to drive a compressor for an air conditioning system. A PSC motor has two windings, a main run winding and a start winding. A start capacitor, whose value is chosen as a compromise between start and run performance, is placed in series with the start winding. This capacitor creates a phase shift the run winding and thus increases both starting and run torque, though it is not optimized for either. When power is applied, both the start winding and the run winding, aided by the phase shift of the capacitor, contribute to the necessary torque to cause initial torque and acceleration. As the motor achieves operating speed, both windings continue to contribute to the motor's torque although the run winding now contributes to a far greater degree due to its lower impedance.
Replacement devices which provide additional starting torque to PSC electric motors have been known for some time. These hard-start devices are added to systems with PSC motors that experienced difficulty starting due to motor or component wear, as well as low line voltage conditions which may hinder or prevent normal starting. These hard-start devices are both mechanical and electronic. A hard-start device typically contains an auxiliary start capacitor of suitable value to provide additional starting torque for a wide range of motor sizes and a switch component to disconnect the start capacitor after the motor has reached a nominal speed. Typically, these hard-start devices are connected in parallel to the existing run capacitor and used two wire connection systems. Hard-start devices of this type rely on either a fixed potential relay or a positive temperature coefficient thermistor to disconnect the start capacitor after the motor has reached a predetermined speed. Other hard-start devices utilize a voltage and/or time dependent circuits to control the switch off of the auxiliary start capacitor.
Where a normally closed relay is used for connecting the auxiliary start capacitor in parallel with the existing start capacitor, problems may arise when voltage is disconnected from the motor. Particularly, when voltage is disconnected from the motor, the relay returns to its normally closed position. Depending on the phase of the AC current, a large voltage from the auxiliary start capacitor may be applied across the closing contacts of the relay thereby welding or otherwise burning the relay contacts.
In addition, prior art hard-start devices which rely on a set point voltage across the run winding to disconnect the auxiliary start capacitor may not function correctly when low line voltage conditions exist. Similarly, time out circuits may prematurely disconnect the auxiliary start capacitor under conditions of low line voltage.